THIS INVENTION relates to cobalt catalysts. It relates in particular to a cobalt catalyst precursor, to a process for preparing a cobalt catalyst precursor, and to a process for preparing a cobalt catalyst.
Processes for preparing cobalt catalysts are, in general, well defined in the literature. For example, U.S. Pat. No. 5,733,839 describes a process for preparing an impregnated Fischer-Tropsch catalyst comprising an alumina carrier and an active component selected from the group consisting of cobalt, iron and mixtures thereof. However, it is an object of the present invention to provide a supported cobalt catalyst having higher productivities than known cobalt catalysts.
Thus, according to a first aspect of the invention, there is provided a cobalt catalyst precursor which includes a catalyst support impregnated with cobalt, with all reducible cobalt being present in the support as supported cobalt oxide of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70.
In other words, according to the first aspect of the invention, there is provided a cobalt catalyst precursor which includes a catalyst support that has been impregnated with cobalt and calcined in such a manner that all reducible cobalt present therein, ie cobalt that is associated with the elements hydrogen and oxygen in the absence of cobalt-support interaction, such as the formation of cobalt aluminates or cobalt silicates, that would decrease its reducibility, is present as the supported cobalt oxide of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70.
Thus, for example, all the reducible cobalt could be present as Co2O3.H2O or CoO(OH), ie where a=2 and b=1. However, instead, the reducible cobalt may be present, for example, as a mixture of Ci3O4 and CoO(OH) or Co2O3.H2O with 45% of the reducible cobalt in the catalyst precursor being present as Co3O4 and 55% of the reducible cobalt being present as CoO(OH) or Co2O3.H2O. This would result in a catalyst precursor in which all the reducible cobalt is present as supported cobalt oxide of formula-unit CoOaHb where a=1,7 and b=0,55. Another example would be a mixture of Co2O3 and CoO(OH) or Co2O3.H2O with 60% of the reducible cobalt being present as Co2O3 and 40% of the reducible cobalt being present as CoO(OH) or Co2O3.H2O. This would result in a catalyst precursor in which all of the reducible cobalt is present as supported cobalt oxide of formula-unit CoOaHb where a=1,7 and b=0,4.
The catalyst precursor may contain between 5 gCo/100 g support and 70 gCo/100 g support, preferably between 20 gCo/100 g support and 50 gCo/100 g support, more preferably between 25 gCo/100 g support and 40 gCo/100 g support.
According to a second aspect of the invention, there is provided a process for preparing a cobalt catalyst precursor, which process includes
in a support impregnation stage, impregnating a particulate porous catalyst support with a cobalt salt, and partially drying the impregnated support; and
in a calcination stage, calcining the partially dried impregnated support to obtain the cobalt catalyst precursor, with the calcination being effected at calcination conditions selected so that all reducible cobalt is present in the support as a supported cobalt oxide of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70.
The resultant cobalt catalyst precursor will, in practice, be reduced to obtain a cobalt catalyst, which will thus have enhanced productivities as compared to catalysts of which Co3O4 is the preferred product of calcination.
According to a third aspect of the invention, there is provided a process for preparing a cobalt catalyst, which process includes
in a support impregnation stage, impregnating a particulate porous catalyst support with a cobalt salt, and partially drying the impregnated support;
in a calcination stage, calcining the partially dried impregnated support to obtain a cobalt catalyst precursor, with the calcination being effected at calcination conditions selected so that all reducible cobalt is present in the support as a supported cobalt oxide of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70; and
in a reduction stage, reducing the cobalt catalyst precursor, to obtain the cobalt catalyst.
The cobalt salt may, in particular, be cobalt nitrate, Co(NO3)2.6H2O.
Any commercially available porous oxide catalyst support, such as alumina (Al2O3), silica (SiO2), titania (TiO2), magnesia (MgO), and silica-alumina, may be used. The support preferably has an average pore diameter between 8 and 50 nanometers, more preferably between 10 and 15 nanometers. The support pore volume may be between 0,1 and 1,0 ml/g, preferably between 0,3 and 0,9 ml/g. The average particle size is preferably between 1 and 500 micrometers, more preferably between 10 and 250 micrometers, still more preferably between 45 and 200 micrometers.
The support may be a protected modified catalyst support, containing, for example, silicon as a modifying component, as described in WO 99/42214, which is hence incorporated herein by reference.
The impregnation of the catalyst support may, in principle, be effected by any known impregnation method or procedure such as incipient wetness impregnation or slurry phase impregnation. However, the impregnation stage may, in particular, comprise a process as described in WO 00/20116, and which is thus incorporated herein by reference. The support impregnation stage may thus involve a 2-step slurry phase impregnation process, -which is dependent on a desired cobalt loading requirement and the pore volume of the catalyst support.
During either of the two slurry phase impregnation steps, a water soluble precursor salt of palladium (Pd), platinum (Pt), ruthenium (Ru), or mixtures thereof, may be added, as a dopant capable of enhancing the reducibility of the cobalt. The mass proportion of the palladium, platinum or ruthenium metal, or the combined mixture of such metals when such a mixture is used, to the cobalt metal may be between 0,01:100 to 0,3:100.
The support impregnation and drying may typically be effected in a conical vacuum drier with a rotating screw or in a tumbling vacuum drier.
In the calcination stage, the calcination may include passing hot air over and around the partially dried impregnated support, thereby drying the impregnated support further by removal of residual moisture present therein; and calcining the resultant dried impregnated support by decomposition of the cobalt salt into decomposition products comprising oxide(s) and any water of hydration, with the decomposition products being released in vapour form, to enhance formation of a supported catalyst precursor containing the supported cobalt oxide of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70.
In particular, since the cobalt salt is cobalt nitrate, the oxide (s) can be nitrogen dioxide, an equilibrium between NO2 and N2O4, as well as nitric oxide (NO).
The process may include diluting the decomposition products that are obtained during the calcination. In other words, formation of the supported cobalt oxide, with the formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70, is enhanced by ensuring dilution of the decomposition products during calcination.
The presence of the supported cobalt oxide phase with the formula-unit CoOaHb, where axe2x89xa7, 7 and bxe2x89xa70, may be determined by using Temperature Programmed Reduction (TPR) as fingerprint technique.
The minimum temperature at which the calcination is performed is the temperature at which decomposition of the cobalt precursor, ie the cobalt salt, starts, while the maximum calcination temperature is the temperature at which the preferred supported cobalt oxide phase with the formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70, is converted to the undesired Co3O4 spinel phase.
The calcination may be performed in any known calcination equipment such as a fluidized bed calciner, a rotary kiln, torbed calciner or a furnace.
In particular, the calcination may be performed in a fluidized bed calciner. Preferably, the calcination is performed in air, at temperatures between 95xc2x0 C. and 400xc2x0 C. The minimum calcination temperature is thus determined by the temperature at which nitrate decomposition starts, ie about 120xc2x0 C., while the maximum calcination temperature is the temperature at which the preferred cobalt oxide with the formula-unit CoOaHb, is converted into the undesired Co3O4 spinel phase. The maximum calcination temperature is typically between 200 and 300xc2x0 C. After calcination, the nitrogen concentration in the catalyst precursor is preferably less than 1,0 m %.
In a preferred embodiment of the invention, the removal of the nitrogen oxide(s) and water during the calcination in air is enhanced, to stabilize the supported cobalt oxide phase of formula-unit CoOaHb, where axe2x89xa71,7 and bxe2x89xa70.
During the calcination in the fluidized bed, the heating rate of the impregnated and dried support and the space velocity of the air may be controlled such that the residual moisture in the support is first driven off, whereafter decomposition of the cobalt nitrate is effected.
Preferably a minimum air space velocity is used in order to attain a maximum relative intrinsic cobalt catalyst activity in respect of the resultant catalyst; however, a maximum heating rate is associated with the minimum space velocity. In particular, the Applicant has found that for a particular support impregnated with cobalt nitrate, there is a minimum air space velocity, 1,0 mn3/(kg Co(NO3)2.6H2O)/h, which must be used in order to attain the maximum relative intrinsic catalyst activity. If a space velocity lower than 1,0 mn3/kg Co(NO3)2.6H2O)/h, is used, the maximum initial activity of the resultant catalyst will thus not be attainable. However, the Applicant has also found that there is then a maximum heating rate of 1xc2x0 C./min, preferably 0,5xc2x0 C./min, and typically between 0,1 and 0,5xc2x0 C./min, associated with the minimum space velocity, 1,0 mn3/(kg Co(NO3)2.6H2O)/h. In other words, if the support heating rate exceeds 1xc2x0 C./min while maintaining a minimum space velocity of 1,0 mn3/(kg Co(NO3)2.6H2O)/h, then this will have a detrimental influence on the initial activity.
However, if a space velocity greater than 1,0 mn3/(kg Co(NO3)2.6H2O)/h is used, say a space velocity of 1000 mn3/(kg Co(NO3)2.6H2O)/h), then there will be a higher maximum or threshold heating rate at which the support can be heated, associated therewith, viz 100xc2x0 C./min. Thus, flash calcination can be applied provided sufficiently high space velocities are employed.
The process may include, in the calcination stage, initially heating the impregnated support until it reaches a calcination temperature, Tc. Thus, the heating up of the support to the calcination temperature, Tc, may be effected while maintaining a space velocity of at least 1,0 mn3/(kg Co(NO3)2.6H2O)/h and an appropriate bed heating rate associated with the space velocity, as described hereinbefore. The process may also include, thereafter, maintaining the support at the calcination temperature, Tc, for a period of time, tc.
The heating rate may be controlled by controlling feed gas (air) pre-heaters through which the air required for fluidization and calcination passes and/or by controlling the calciner wall temperature. While the heating rate up to Tc can be linear, it is believed that enhanced activity may be obtainable if the heating rate is non-linear in order to tailor the release profiles of the nitrogen oxide(s) and water.
The period of time, tc, for which isothermal calcination is applied at the calcination temperature, Tc, may be between 0,1 and 20 hours, provided that the nitrogen content of the calcined catalyst is less then 1,0 m %.
The partially dried impregnated support from the support impregnation stage is preferably not stored and not heated or cooled prior to the subsequent fluidized bed calcination stage so that it passes into the fluidized bed calcination stage at substantially the same temperature at which it leaves the support impregnation stage. Thus, the partially dried impregnated support typically leaves the support impregnation stage at a temperature between 60xc2x0 C. and 95xc2x0 C., typically at around 75xc2x0 C., and enters the fluidized bed calcination stage at about the same temperature, and without storage thereof between the stages. The fluidized bed calcination stage is thus preferably provided by a fluidized bed calciner connected directly to the vacuum drier.
The fluidized medium employed in the calciner is thus the air which is required for the calcination, and the linear air velocity through the calciner must naturally be sufficiently high to ensure proper fluidization.
The invention extends also to a catalyst when obtained by the process according to the third aspect of the invention, or when obtained by reducing the catalyst precursor of the first aspect of the invention or the catalyst precursor obtained by the process according to the second aspect of the invention.
The process is particularly suited to preparing a cobalt slurry phase Fischer-Tropsch catalyst, ie a catalyst suitable for catalyzing the conversion of a synthesis gas comprising carbon monoxide and hydrogen to hydrocarbon products, at elevated temperature and pressure.
The invention will now be described by way of example with reference to the following non-limiting examples and the accompanying diagrammatic drawings.